In measuring the content of an object element or elements in a sample with a flameless atomic absorption spectrophotometer, a known amount of sample is put in a heating tube (often a graphite tube is used), and the tube is heated to a high temperature to atomize the sample. A light is passed through a cloud of the atomized sample and the absorbing ratio (or the transmissivity) of the light at a specific wavelength or wavelengths corresponding to the object element is measured.
When metallic element or elements of a sample is measured and the sample includes water or organic component, the water or organic component is also vaporized in heating the tube and their vapors interfere with the absorption measurement of the object metallic element or elements. In order to avoid the interference, the heating program as shown in FIG. 1 (top) is used. First, the tube is heated at about 100.degree. C. for several tens of seconds to vaporize the water content of the sample (drying). Then the tube is heated at 100.degree.-1000.degree. C. for several tens of seconds to vaporize the organic content of the sample (burning). After the water and organic contents are thus removed, the tube is heated to a proper high temperature (1000.degree.-3000.degree. C.) to atomize the object component (mainly metallic elements), on which the absorbing ratio is measured. The time needed to atomize the object component is several seconds.
In the drying and burning stages of the heating program, as shown in FIG. 9, a cloud 84 of water vapor or organic vapor is generated in the tube 81. Though some part of the cloud 84 goes out of the tube 81 through the small sampling hole 82 at the top of the tube 81, a large part of the cloud 84 expands along the optical path 83 toward the open ends of the tube 81 and lingers within the tube 81 until the atomizing stage. In a conventional flameless atomic absorption spectrophotometer, therefore, as shown in FIG. 10, a pair of transparent windows (often quartz plates) 95a and 95b are provided at the ends of electrodes 94a and 94b clamping and enclosing the tube 81, and inert gas (Ar gas, N.sub.2 gas, etc.) is introduced within the tube 81 from the both ends of the tube 81 through passages (inner gas passages) 96a and 96b respectively provided at ends of the electrodes 94a and 94b. By such measures, the cloud 84 of water or organic vapor generated at the drying or burning stage is promptly expelled out of the tube 81 from the sampling holes 82 and 98 and the deleterious cloud 84 does not expand toward the ends of the tube 81. Another pair of passages (outer gas passages) 97a and 97b are provided in the electrodes 94a and 94b to introduce inert gas into the space between the tube 81 and the electrode envelope 94a and 94b in order to protect the graphite tube 81 from depletion by oxidization. The outer gas (gas introduced in the space between the tube 81 and the electrode envelope 94a and 94b) is drained from the gap between the two electrodes 94a and 94b.
A problem of the conventional flameless atomic absorption spectrophotometer is that a part of the atomized sample escapes through the sampling hole 82 to the outside of the tube 81 (or out of the optical path 83 of the measurement light). This is because the cloud of atomized sample cannot expand along the optical path 83 since the pressure within the tube 81 rises quickly at the atomizing stage and the ends of the optical path 83 (i.e., the ends of the electrode envelope 94a and 94b) are shut by the windows 95a and 95b.
Another problem of the conventional flameless atomic absorption spectrophotometer is that the windows 95a and 95b are not completely transparent to the measurement light and absorb some amount of light passing through the atomized sample. It is especially critical when the measurement uses light of shorter wavelength such as less than 200 nm. For example, 10% of the light is lost by a quartz window at the wavelength of .lambda.=190 nm. These problems lead to a poorer S/N ratio in the measurement.